Reading memory data

ABSTRACT

A circuit includes a memory array comprising K number of rows. The circuit further including a reference column. The reference column includes M cells of a first cell type configured to provide a first leakage current, K-M cells of a second cell type different from the first cell type, the K-M cells are configured to provide a second leakage current, and a reference data line connected to the cells of the first cell type and the cells of the second cell type. The circuit further includes a sensing circuit configured to determine a value stored in a memory cell of the memory array based on a voltage of the reference data line.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/908,670,entitled “Reading Memory Data” filed on Oct. 20, 2010, the entirety ofwhich is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is related to reading memory data on a data line.

BACKGROUND

In various approaches, data from a bit cell (e.g., a memory cell) isread by detecting the logic level at a corresponding read bit line(e.g., a read bit line RBL). A P-type Metal Oxide Silicon transistor(PMOS transistor) implemented as a feedback keeper is used to compensatethe leakage current from unselected bit cells and thus improves “readone” operations, e.g., reading a logic high level (a High), at read bitline RBL. In “read zero” situations, e.g., reading a logic low level (aLow) at read bit line RBL, however, the PMOS keeper slows down the RBLdischarge speed. In some situations, if the PMOS keeper sinks a lot ofcurrent (e.g., the PMOS transistor has a high conductivity) and/or thecell current is not high enough, the cell current cannot discharge(e.g., pull) read bit line RBL from a High to a Low. As a result, theread operation fails. Variations in the semiconductor manufacturingprocess also cause the cell current to vary at different voltage andtemperature conditions, which also increases the failure rate when theread bit line RBL is read.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of one or more embodiments of the disclosure are set forthin the accompanying drawings and the description below. Other featuresand advantages will be apparent from the description, drawings, andclaims.

FIG. 1 is a diagram illustrating a column of a memory array, inaccordance with some embodiments.

FIG. 2 is a diagram of a memory cell of the memory array in FIG. 1, inaccordance with some embodiments.

FIG. 3 is a diagram of a circuit illustrating reading data on a read bitline of the memory array in FIG. 1, in accordance with some embodiments.

FIGS. 4A and 4B are graphs of waveforms illustrating the operation ofthe circuit in FIG. 3, in accordance with some embodiments.

FIG. 5 is a block diagram of a reference column used in the circuit ofFIG. 3, in accordance with some embodiments.

FIG. 6 is a diagram illustrating a first cell type used in the referencecolumn of FIG. 5, in accordance with some embodiments.

FIG. 7 is a diagram illustrating a second cell type used in thereference column of FIG. 5, in accordance with some embodiments.

FIG. 8 is a flowchart illustrating a method of operation of the circuitin FIG. 5, in accordance with some embodiments.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Embodiments, or examples, illustrated in the drawings are disclosedbelow using specific language. It will nevertheless be understood thatthe embodiments and examples are not intended to be limiting. Anyalterations and modifications in the disclosed embodiments, and anyfurther applications of the principles disclosed in this document arecontemplated as would normally occur to one of ordinary skill in thepertinent art. Reference numbers may be repeated throughout theembodiments, but they do not require that feature(s) of one embodimentapply to another embodiment, even if they share the same referencenumber.

Some embodiments can have one or a combination of the following featuresand/or advantages. In some embodiments, the single-end read circuitincludes a leakage tracking column so that the PMOS feedback keeper iseliminated, which improves the read speed, and prevents data contentionbetween the cell current and that of the PMOS keeper.

FIGS. 3 and 4 are a circuit and waveforms for a circuit that is adaptedto read a memory cell or cells that have either high or low leakage. Thecircuit adapts the method of reading a bit line depending upon theleakage on a reference bit line.

FIGS. 5, 6 and 7 are circuits adapted to provide a memory cell withknown leakage. In some embodiments, the circuit of FIG. 3 is combinedwith one or more of the circuits of FIGS. 5, 6 and 7 to operate with apredetermined leakage. In other embodiments, the circuit of FIG. 3 isnot combined with the circuits of FIGS. 5, 6 and 7.

Exemplary Memory Circuits

FIG. 1 is a diagram of a column 100 of a memory array in accordance withsome embodiments. In this illustration, column 100 includes K memorycells MC, e.g., memory cells MC-1 to MC-K. The number of memory cells MCin column 100, e.g., the value of K, varies, depending on theconfiguration of the memory cell array, including, for example, a valueof 16, 32, 64, etc. A read bit line, e.g., read bit line RBL is coupledto the K number of memory cell MCs in column 100. In some embodiments, aread word line RWL corresponding to a memory cell MC is coupled to everymemory cell in a row (not shown). For illustration, FIG. 1 shows readword line RWL-1 and RWL-K corresponding to row 1 and row K,respectively.

In some embodiments, a memory cell MC includes six transistors, and iscommonly called a 6T cell, which is known in the art. Transistors N1 andN2 corresponding to a memory cell MC are used in reading data in amemory cell MC. The 6T cell together with the corresponding pair oftransistors N1 and N2 are commonly called an eight-transistor (8T)memory cell.

FIG. 2 is a diagram of an 8T memory cell, e.g., memory cell 200,comprising a memory cell MC-1 coupled to transistor N1-1 and N2-1, inaccordance with some embodiments. Memory cell 200, via transistor N3 andN4, is also coupled to a write word line, e.g., WWL, and a pair of writebit lines, e.g., write bit line WBL-1 and WBLB-1 for writing. PMOStransistors P1 and P2, and NMOS transistors N5 and N6 form a crosslatch, e.g., cross latch Xltch for memory cell MC-1 or for memory cell200. Storage nodes NO and NOB store data for memory cell 200.

In some embodiments, when a cell MC of the memory array is accessed(e.g., for reading), a plurality of memory cells MC in a row is accessed(e.g., the accessed memory cells AMC). Memory cells in the memory arrayother than the accessed memory cells AMC are called un-accessed memorycells UAMC. A column having an accessed memory cell AMC is called anaccessed column AC. A column having all un-accessed memory cells UAMC iscalled an un-accessed column UAC. In some embodiments, for an accessedcolumn AC, there is a fix number (e.g., 3, 7, 15, etc.) of un-accessedcolumns.

Before a memory cell MC is accessed for reading, the corresponding readbit line RBL is pre-charged to a High. The term “pre-charged” (versus“charged”) is used to indicate that read bit line RBL is charged (e.g.,brought to a High) prior to reading. When a memory cell MC is accessedfor reading, the corresponding read word line RWL is activated and thedata is read by detecting the logic level at the corresponding read bitline RBL. If node NO stores a Low then transistor N2-1 is off, e.g.,acting as an open circuit. As a result, read bit line RBL isdisconnected from transistors N1-1 and N2-1, and thus remains a High atthe pre-charge level. In contrast, if node NO stores a High, transistorN2-1 is on, which together with the then on transistor N1-1 pulls readbit line RBL to the voltage level at the source of transistor N2-1,which is ground, or Low. The current flowing on read bit line RBL whileread bit line is read is called read current Iread. The current flowingfrom read bit line RBL through transistor N2-1 and N2-1 through groundwhen node NO stores a High is, however, significantly more than thecurrent flowing through read bit line RBL when node NO stores a Low, andis the worst-case read current. Current Irmax shown in FIG. 2illustrates the read current when node NO stores a High and RBL iseventually pulled to Low, which is the worst-case read current.

For illustration, memory cell MC-1 is accessed. As a result, read wordline RWL-1 is activated, which turns on transistor N1-1. If the datastored in node NO of memory cell MC-1 High, the gate of transistor N2-1is High, and transistor N2-1 is turned on. As a result, transistors N1-1and N2-1, being on, pull (e.g., discharge) the voltage level at read bitline RBL to the voltage level at the source of transistor N2-1, which isground. In contrast, if the data stored in node NO of memory cell MC-1is Low, the gate of transistor N2-1 is Low, and transistor N2-1 is off,which acts as an open circuit. As a result, read bit line RBL remainsHigh. The data at read bit line RBL is processed (e.g., inverted) toreflect (e.g., match) the data stored in node NO.

In some embodiments, even if memory cell 200 is not accessed, currentleaks from read bit line RBL through transistor N1-1 and N2-1, and theleakage current is worst when node NO stores a High. The leakage currentwithout being controlled can cause a read error on read bit line RBL. Insome embodiments, mechanisms are provided to prevent the erroneous readdue to the leakage current in un-accessed memory cells in a column whena memory cell is read.

In FIG. 2, the gate of transistor N2-1 (e.g., gate GN2) is coupled tonode NO, but gate GN2 can be coupled to node NOB, and the operation ofnode NO applies to the operation of node NOB as would be recognizable bya person of ordinary skill in the art.

Exemplary Read Circuit

FIG. 3 is a diagram of a read circuit 300, in accordance with someembodiments. In this illustration, circuit 300 is used to detect data ona read bit line RBL with reference to the data on a reference read bitline RERBL. Circuit 300 is adapted to read a memory cell or cells suchas the memory cells of FIGS. 1 and 2. The circuit adapts the method ofreading the bit line RBL depending upon the leakage on the reference bitline RERBL.

Here, when the data in any memory cell of a column, e.g., memory cellMC-1 to MC-K in column 100, is accessed for reading, circuit 300provides the read data of that memory cell MC on output Out, which isthen processed to match the data stored in memory cell MC. Forillustration, memory cell MC-1 is accessed, but the principles describedin this document apply to any other memory cell. Memory cells MC-2 toMC-K in column 100 are un-accessed (e.g., unselected) memory cells.

Latch Ltch latches the data at input D to output Q at the rising edge ofclock CLK. The data on read bit line RBL is fed to input D of latchLtch. Reference read bit line (RERBL) is used to as a clock to latch(e.g., to clock) the data on read bit line RBL from input D to output Qof latch Ltch. For example, when reference bit line REBL turns from aHigh to a Low, output of inverter INV, e.g., output Oinv, turns High,and provides a rising edge to clock the data at input D (e.g., the dataon read bit line RBL) to output Q. At the same time, because output Oinvis IIigh, the SEL input of multiplexer MUX is activated to select outputQ (e.g., the latched data) to appear at output Out. Output Out thusreceives the latched data as shown in path (2). If reference read bitline RERBL, however, does not turn from a High to a Low (e.g., remainsHigh), there is no rising edge of the clock to latch the data at input D(e.g., the data on read bit line RBL). At the same time, output Oinv isinverted from reference read bit line RERBL to be Low, which provides aLow to input SEL of multiplexer MUX. As a result, the data on read bitline RBL that is at the Low input (e.g., the “0” input) of multiplexerMUX is selected as the data output at output Out. In other words, thedata on read bit line RBL is selected as the output at output Out, whichis shown as path (1). In some embodiments, reference read bit line RERBLis discharged (e.g., pulled towards a Low) by current Iref, which, forexample, is generated by circuit 500 in FIG. 5.

In FIG. 3, inverter INV is used to invert reference read bit line RERBLto adapt to the positive edge trigger of the CLK input of latch Ltch. Inembodiments, where input clock CLK is negative edge trigger referenceread bit line RERBL is directly used to trigger latch Ltch withoutinverter INV. Further, latch Ltch and multiplexer MUX are used forillustration, other circuits that implement the following operation arewithin the scope of various embodiments. For example, when referenceread bit line RERBL does not trip (e.g., does not change the logic levelfrom a High to a Low), the data on read bit line RBL is provided as theoutput data, but when reference read bit line RERBL trips, the data onread bit line RBL at the trip point time is selected as the output.

Exemplary Wave Forms

In some embodiments, Latch Ltch includes an inverter, e.g., an LINV (notshown) at the input D. Each of inverter INV and LINV determines whetherits input is High or Low based on a trip point, below which the data isLow, and above which the data is High. For illustration, the trip pointof inverter LINV is called trip point Ltrip, and the trip point ofinverter INV is called Trip. In some embodiments, trip point Trip andtrip point Ltrip are substantially the same (e.g., equal).

In some embodiments, while read bit line RBL is read, a current, e.g.,current Iref is generated to effect the voltage level of reference readbit line RERBL, based on which the logic level of read bit line RBL isdetected, e.g., as latched or unlatched through latch Ltch. Further, thecurrent flowing through read bit line RBL is called read current Iread.Reference current Iref and current Iread cause reference read bit lineRERBL and read bit line RBL to be discharged, respectively. The term“discharge” refers to the voltage level on reference read bit line REBLor read bit line RBL being pulled down (e.g., from a High towards aLow).

FIGS. 4A and 4B are graphs of waveforms illustrating the behavior ofreference read bit line RERBL and read bit line RBL, in accordance withsome embodiments. In both FIGS. 4A and 4B, reading or detecting thelogic level of a read bit line RBL occurs after read word line RWL isHigh for a period of time (e.g., after a set up time with respect to therising edge of read word line RWL). Further, because in someembodiments, trip point Trip and trip point Ltrip are substantially thesame, “Trip” is used to indicate the trip point for both trip point Tripand Ltrip.

In FIG. 4A, the leakage current Ileak is little (e.g., a light leakagecurrent situation). In this situation, when the data on read bit lineRBL is High (shown as read one or R1), current Iref is higher thancurrent Iread. As a result, reference read bit line RERBL is pulledlower than read bit line RBL, but reference read bit line RERBL does nottrip. Stated another way, both read bit line RBL and reference read bitline RERBL stay High, despite the leakage current. In contrast, when thedata on read bit line RBL is Low (e.g., shown as read zero or R0),current Iread is higher than current Iref. As a result, read bit lineRBL is pulled down faster than reference read bit line RERBL beingpulled down. Read bit line RBL eventually trips, e.g., turns from a Highto a Low. In effect, reference read bit line RERBL remains High, butread bit line RBL turns Low. In some embodiments, when leakage currentIleak is light, the data on read bit line RBL is directly used as theread data, e.g., shown as path (2) in FIG. 3.

In FIG. 4B, leakage current Ileak is significant. In this situation,when the data on read bit line RBL is High (shown as R1), current Irefis higher than current Iread, reference read bit line RERBL is pulledlower than read bit line RBL. Both read bit line RBL and reference readbit line RERBL eventually trip, i.e., both read bit line RBL andreference read bit line RERBL turn Low. Reference read bit line RERBL,however, turns Low prior to read bit line RBL turning Low. In someembodiments, at the time reference read bit line RERBL turns Low (e.g.,time t1), the data on read bit line RBL is latched and the latched datais used as the output at output Out in FIG. 3. Because at time t1, readbit line RBL has not turned Low, i.e., read bit line RBL is still High,latching read bit line RBL is latching High data, and thus results in aHigh at output Out, as shown as path (1) in FIG. 3.

In contrast, when the data on read bit line RBL is Low (shown as R0),current Tread is higher than current Iref. As a result, read bit lineRBL is pulled down faster than reference read bit line RERBL beingpulled down, and both read bit line RBL and reference read bit lineRERBL eventually turn Low. Even though both reference read bit lineRERBL and read bit line RBL turn Low, read bit line RBL turns Low priorto reference bit line RERBL turning Low. In some embodiments, at thetime reference read bit line RERBL turns Low (e.g., time t2), the readbit line RBL is latched and the latched data is used as the output atoutput Out in FIG. 3. Because at time, t2, read bit line RBL has turnedLow, latching read bit line RBL is latching a Low data and thus resultsin a Low at output Out, as shown as path (1) in FIG. 3.

In some embodiments, current Iref is configured such that current Irefis between the current (e.g., current Irmax) when read bit line RBL ispulled from a High to a Low (e.g. by transistors N1 and N2) and theworst case of the leakage current Ileak in a column to be read. Further,current Ileak is worst when nodes NO of un-selected (e.g., un-accessed)memory cells in the read column store High data. Current Irmax is thecurrent flowing from read bit line RBL through transistors N1 and N2while read bit line RBL is being read, and node NO stores a High.

Exemplary Reference Circuits

FIGS. 5, 6 and 7 are circuits adapted to provide memory cells with aknown reference leakage. FIG. 5 is a diagram of a column 500illustrating a reference column, in accordance with some embodiments.Column 500 includes two different types of cells, e.g., cell C1 and cellC2 (collectively called cell C). Each of cells C1 and C2 sinks a currentIC1 and a current IC2, respectively, providing the known referenceleakage. In some embodiments, the total number of cells C in column 500is the same as the total number of memory cells MC in column 100. Forexample, if column 100 has K number of memory cells MC, then column 500has K number of cells C. Of the K number of cells C in column 500, thereare two cells C1 (e.g., cells C1-1 and C1-2) and K−2 number of cells C2(cell C2-1 to C2-(K−2). A cell C is modified from an 8T memory cell 200,in which a cell C has similar transistors, e.g., transistors P1, P2, N3,N4, N5, N6, N1 and N2, but with different configurations. Column 500 isused to generate current Iref, which, in some embodiments, is the sum ofall currents flowing from reference read bit line RERBL to cells C. As aresult, current Iref is the sum of currents IC1-1, IC1-2, and IC2-1 toIC2-(K−2), corresponding to cells C1-1, C1-2, C2-1 to C2-(K−2). In someembodiments, current IC1-1 is the same (e.g., substantially the same) ascurrent IC1-2. Similarly, each of current IC2-1 to IC2-(K−2) are thesame (e.g., substantially the same) as each other. In effect, currentIref is the sum of two currents of value IC1 and K−2 currents of valueIC2.

In some embodiments, one reference column 500 is generated in a memoryarray. Each time a column is selected for reading, reference column 500is used with the selected column to be read as illustrated in FIG. 3.

FIG. 6 is a diagram 600 illustrating two cells C1 (e.g., cell C1-1 andcell C1-2), in a column 500, in accordance with some embodiments. Tosimplify the drawings, each cross latch Xltch of each cell C1 thatincludes transistors P1, P2, N5 and N6 is shown as two inverters IN1 andINV2, and is recognizable by a person of ordinary skill in the art.Further, only the details of one cell C1 are labeled.

The gates of transistors N1, N2, N3, and N4 of cells C1-1 and C1-2 arecoupled together and to reference word line REWL, which, when activated,is High.

The drains of transistors P1 and N5 are no longer coupled together. NodeNP1, which couples the drain of transistor P1 and the drain oftransistor N3 and is set to a High (e.g., voltage Vdd) while node NN5,which is the drain of transistor N5, is floating (e.g., not connected toanother circuit).

In general, node NO, which is the gate of transistor N2 and also theoutput of inverter INV1, is Low because node NP1 is High. Afterreference word line REWL is activated that turns on transistors N3 andN4, node NO is raised by a voltage (e.g., voltage Vraise), which isabout 100 mV-200 mV in some embodiments. Because of Vraise at the gateof transistor N2, transistor N2 is on “slightly,” e.g., transistor N2 isnear a conduction state to be completely on, which causes current IC1 toflow (e.g., to leak) from reference bit line REBL through transistor N1and N2 to ground as shown.

Reference write bit lines REWBL and REWBLB are set to High (e.g.,voltage Vdd) so that when reference word line is High, the High onreference write bit lines REWBL and REWBLB generate a current flowingfrom reference write bit lines REWBL and REWBLB into the storage nodesof cells C1 and thus induces higher leakage for reference read bit lineRERBL.

FIG. 7 is a diagram 700 of a cell C2, in accordance with someembodiments. Compared with an 8T-cell 200, cell C2 has similarcomponents but with difference configurations.

In cell C2, the gates of transistors N1, N3, and N4 are grounded (e.g.,set to Vss, Low), causing transistor N1, N3, and N4 to turn off. NodeNN5 is floating (e.g., not coupled to any circuit).

Node NP1 is grounded, which causes node NO, the gate of transistor N2,to be High and transistor N2 to turn on. Because transistor N2 is on,current IC2 flows (e.g., leaks) from reference read bit line RERBLthrough transistor N1 and N2 as shown.

In FIGS. 5-7, current Iref comprises two currents IC1 and K−2 currentsIC2, which is greater than current Ileak (the leakage current in acolumn due to un-accessed cells) and less than current Irmax.

FIGS. 5-7 show circuits illustrating an implementation such that currentIref is in between current Irmax and current Ileak. Other circuitsserving the same function are within the scope of various embodiments.

Exemplary Method

FIG. 8 is a flowchart 800 illustrating a method of reading a memory cellMC using circuit 500 in FIG. 5, in accordance with some embodiments. Forillustration, the data in memory cell MC-1 is read, but the principledescribed herein applies to other memory cell.

In step 805, read bit line RBL and reference read bit line REBL arepre-charged to a High.

In step 810, reference word line REWL is activated to turn ontransistors N5 and N6 in cells C1-1 and C1-2 of column 500 and thusgenerate current Iref. At the same time, read word line WL-1 is also isactivated to select memory cell MC-1 (and other memory cell MC in row 1)for reading. For illustration, reading the data in memory cell MC-1 isillustrated, but the reading principles apply to other accessed memorycells.

In step 815, the voltage level on read bit line RBL changes based on thedata stored in node NO and current Tread. At the same time, the voltagelevel on reference bit line REBL changes based on current Ileak. Thedata on read bit line RBL is reflected at output Out of circuit 300,latched or unlatched. For example, in the case of a light leakagesituation, reference read bit line RERBL does not trip, output Oinv thatappears at input SEL of multiplexer MUX is Low, which causes the data onread bit line RBL at the input 0 of multiplexer MUX to be selected asoutput at output Out. In the case of heavy leakage situations, thereference read bit line RERBL trips to a Low causing output Oinv to beHigh, which serves as a clock and latches the data on read bit line RBLat the D-input of latch Ltch to output Q. At the same time, the SELinput of multiplexer MUX (e.g., which is the same as output Oinv) isHigh causing the latched data on read bit line at input “1” (e.g., inputHigh) of multiplexer to be selected as output at output Out.

In step 820, the data at output Out is processed to match the datastored in node NO. For example, if the data stored in node NO is Low,read bit line RBL is High, which appears as High at output Out, and isthen inverted to match the Low data stored in node NO. Similarly, if thedata stored in node NO is High, read bit line RBL is Low, which appearsas Low at output Out, and is then inverted to match the High stored innode NO.

Some embodiments regard a circuit including a memory array comprising Knumber of rows. The circuit further including a reference column. Thereference column includes M cells of a first cell type configured toprovide a first leakage current, K-M cells of a second cell typedifferent from the first cell type, the K-M cells are configured toprovide a second leakage current, and a reference data line connected tothe cells of the first cell type and the cells of the second cell type.The circuit further includes a sensing circuit configured to determine avalue stored in a memory cell of the memory array based on a voltage ofthe reference data line.

Some embodiments regard a circuit including a memory array including Knumber of rows of memory cells. The reference column includes M cells ofa first cell type configured to provide a first leakage current, K-Mcells of a second cell type different from the first cell type, and theK-M cells are configured to provide a second leakage current. Thereference column further includes a reference data line connected to thecells of the first cell type and the cells of the second cell type. Thereference column is configured to provide a reference current on thereference data line, the reference current being equal to a sum of atotal of the first leakage current for each of the M cells of the firstcell type and a total of the second leakage current for each of the K-Mcells of the second cell type. The circuit further includes a sensingcircuit configured to determine a value stored in a memory cell of thememory array based on the reference current.

Some embodiments regard a method of reading data stored in a memory cellof a column of memory cells. The method includes activating the memorycell, wherein the column of memory cells comprises K rows, and a datavoltage results from the activating the memory cell. The method furtherincludes activating a reference column, the reference column comprisingM cells of a first cell type, K-M cells of a second cell type differentfrom the first cell type, and a reference data line connected to thecells of the first cell type and the cells of the second cell type,wherein a reference voltage results from activating the referencecolumn. The method further includes determining the data stored in thememory cell based on the reference voltage and the data voltage. Theabove methods show exemplary steps, but they are not necessarilyperformed in the order shown. Steps may be added, replaced, changedorder, and/or eliminated as appropriate, in accordance with the spiritand scope of disclosed embodiments.

A number of embodiments have been described. It will nevertheless beunderstood that various modifications may be made without departing fromthe spirit and scope of the disclosure. For example, the varioustransistors being shown as a particular dopant type (e.g., NMOS andPMOS) are for illustration purposes, embodiments of the disclosure arenot limited to a particular type, but the dopant type selected for aparticular transistor is a design choice and is within the scope ofembodiments. The logic level (e.g., low or high) of the various signalsused in the above description is also for illustration purposes, variousembodiments are not limited to a particular level when a signal isactivated and/or deactivated, but, rather, selecting such a level is amatter of design choice. Read bit lines (e.g., RERBL, RBL, etc.), readword lines (e.g., REWL, RWL, etc.), write bit lines (e.g., REWBL,REWBLB, WBL, WBLB, etc.), write word line (e.g., REWL, WL, etc.) aredata lines (e.g., they carry data).

What is claimed is:
 1. A circuit comprising: a memory array comprising Knumber of rows of memory cells; a reference column, the reference columncomprising: M cells of a first cell type configured to provide a firstleakage current, K-M cells of a second cell type different from thefirst cell type, the K-M cells are configured to provide a secondleakage current, and a reference data line connected to the cells of thefirst cell type and the cells of the second cell type; and a sensingcircuit configured to determine a value stored in a memory cell of thememory array based on a voltage of the reference data line.
 2. Thecircuit of claim 1, wherein the cells of the first cell type comprise: across latch comprising a first p-type transistor, a second p-typetransistor, a first n-type transistor and a second n-type transistor,wherein a source of the first p-type transistor and a source of thesecond p-type transistor are connected to a supply voltage, a drain ofthe first n-type transistor is connected to a drain of the first p-typetransistor, and a drain of the second n-type transistor is floating. 3.The circuit of claim 2, wherein a drain of the second p-type transistoris connected to a drain of a pass gate transistor.
 4. The circuit ofclaim 2, wherein the cells of the first cell type further comprise: afirst pass gate transistor, wherein a first terminal of the first passgate transistor is connected to a first reference word data line, asecond terminal of the first pass gate transistor is connected to thecross latch, and a gate of the first pass gate transistor is connectedto an activation line; and a second pass gate transistor, wherein afirst terminal of the second pass gate transistor is connected to asecond reference word date line, a second terminal of the second passgate transistor is connected to the cross latch, and a gate of thesecond pass gate transistor is connected to the activation line.
 5. Thecircuit of claim 2, wherein the cells of the first cell type furthercomprise: a first transistor, wherein a first terminal of the firsttransistor connected to the reference line and a gate connected to anactivation line; and a second transistor, wherein a first terminal ofthe second transistor is connected to a second terminal of the firsttransistor, a second terminal of the second transistor is connected to areference voltage and a gate of the second transistor is connected tothe cross latch.
 6. The circuit of claim 1, wherein the cells of thesecond cell type comprise: a cross latch comprising a first p-typetransistor, a second p-type transistor, a first n-type transistor and asecond n-type transistor, wherein a source of the first p-typetransistor and a source of the second p-type transistor are connected toa supply voltage, a drain of the first n-type transistor is connected toa drain of the first p-type transistor, a drain of the second p-typetransistor is connected to a reference voltage and a drain of the secondn-type transistor is floating.
 7. The circuit of claim 6, wherein thecells of the second cell type further comprise: a first pass gatetransistor, wherein a first terminal of the first pass gate transistoris connected to a first reference word data line, a second terminal ofthe first pass gate transistor is connected to the cross latch, and agate of the first pass gate transistor is connected to the referencevoltage; and a second pass gate transistor, wherein a first terminal ofthe second pass gate transistor is connected to a second reference worddate line, a second terminal of the second pass gate transistor isconnected to the cross latch, and a gate of the second pass gatetransistor is connected to the reference voltage.
 8. The circuit ofclaim 6, wherein the cells of the second cell type further comprise: afirst transistor, wherein a first terminal of the first transistorconnected to the reference line and a gate connected to the referencevoltage; and a second transistor, wherein a first terminal of the secondtransistor is connected to a second terminal of the first transistor, asecond terminal of the second transistor is connected to a referencevoltage, a gate of the second transistor is connected to the crosslatch, and a node of the cross latch connected to the gate of the secondis at the supply voltage.
 9. A circuit comprising: a memory arraycomprising K number of rows of memory cells; a reference column, thereference column comprising: M cells of a first cell type configured toprovide a first leakage current, K-M cells of a second cell typedifferent from the first cell type, the K-M cells are configured toprovide a second leakage current, and a reference data line connected tothe cells of the first cell type and the cells of the second cell type,wherein the reference column is configured to provide a referencecurrent on the reference data line, the reference current being equal toa sum of a total of the first leakage current for each of the M cells ofthe first cell type and a total of the second leakage current for eachof the K-M cells of the second cell type; and a sensing circuitconfigured to determine a value stored in a memory cell of the memoryarray based on the reference current.
 10. A method of reading datastored in a memory cell of a column of memory cells, the methodcomprising: activating the memory cell, wherein the column of memorycells comprises K rows, and a data voltage results from the activatingthe memory cell; activating a reference column, the reference columncomprising M cells of a first cell type, K-M cells of a second cell typedifferent from the first cell type, and a reference data line connectedto the cells of the first cell type and the cells of the second celltype, wherein a reference voltage results from activating the referencecolumn; and determining the data stored in the memory cell based on thereference voltage and the data voltage.